More than 2000 primary total knee replacements (TKR) are performed in Hong Kong each year and more than 10 000 patients are on the waiting list for TKR at public hospitals. With an ever-increasing waiting list, joint replacement centres with high surgery volume have been set up in public hospitals. More such centres are planned in the future to tackle the ageing population and rising demand. As one of the most popular elective ‘ultra-major’ surgeries, how safe is primary TKR?

According to various knee replacement registries, 30-day mortality of TKR ranges from 0.2% to 0.4%, 90-day mortality 0.4% to 0.7%, and 1-year mortality 1% to 2%.1 2 3 4 5 6 7 8 Risk factors for post-TKR mortality include age at operation, male sex, too high or low body mass index, American Society of Anesthesiologists (ASA) class 3 to 4, presence of co-morbidities, and simultaneous bilateral surgery.1 2 3 4 6 7 8 9 10 11 12 There are a lack of similar data for the Asian population, however, and the risk of mortality in a high-volume hospital has not been described locally. The aims of the study were to review the mortality of primary TKR in Hong Kong and to identify risk factors of post-TKR mortality in a high-volume hospital.

All primary TKR performed in public hospitals (Hospital Authority) and all primary TKR at the authors’ institute (Yan Chai Hospital, YCH) from October 2011 to September 2014 were reviewed. Data retrieval for all public hospitals was performed with the Clinical Data Analysis and Reporting System. Procedure code for retrieval was “81.54 TOTAL KNEE REPLACEMENT”. Data of patients at our institute were retrieved additionally with Clinical Management System of the Hospital Authority. The medical records of all deceased cases before September 2015 were reviewed.

The primary outcome measures were 30-day, 90-day, and 1-year mortality. Correlation coefficient between 30-day, 90-day, and 1-year mortality and annual surgery volume of all public hospitals was calculated with Pearson test. From data of YCH, comparisons were made between the mortality of unilateral TKR and simultaneous bilateral TKR. Case-control analysis was performed for possible risk factors of primary TKR total mortality. A control group included non-mortality cases of all simultaneous bilateral TKR in the same period and all unilateral primary TKR performed from October 2012 to March 2013. The latter period was chosen to allow a 1-year ‘run-in’ time for the newly established joint replacement centre that commenced operation in October 2011. All bilateral cases were used due to their relative scarcity. Chi squared test and Fisher’s exact test were used for univariate analysis, and multiple logistic regression was used for multivariate analysis. Final model for multiple logistic regression was identified by backward elimination. A P value of <0.05 was considered statistically significant.

There were 6588 primary TKR in 15 public hospitals and 1184 primary TKR at YCH (1095 unilateral and 89 bilateral). The 30-day, 90-day, and 1-year mortality was 0% (n=0), 0.08% (n=1), and 0.34% (n=4) for YCH and 0.1% (n=8), 0.2% (n=16), and 0.7% (n=48) for all public hospitals (YCH inclusive), respectively (Table 1). There was no correlation between hospital surgery volume and 30-day, 90-day, or 1-year mortality among the 15 public hospitals (R=0.151, P=0.578; R=0.031, P=0.910; R=0.032, P=0.972, respectively). For cases at YCH, the mean follow-up time was 12.8 (range, 4-38) months, the mean operation-to-death interval was 21 (1-35) months, and the mean age at death was 78 (70-87) years. Main causes of death were malignancy (50%) and pneumonia (21%) [Table 2]. Significant predictors of total mortality identified by univariate analysis included age at operation, preoperative range of motion (ROM), and ASA class 3; the former two were also confirmed by the final model of multivariable analysis (Table 3). The mean age at operation was 76 versus 68 years for mortality and non-mortality cases while the mean preoperative ROM was 95 versus 108 degrees, respectively. Body mass index, co-morbidities, deep vein thrombosis (DVT) prophylaxis, and postoperative DVT did not differ significantly between the two groups. Preoperative Western Ontario and McMaster Universities Arthritis Index (WOMAC), Knee Society score (KSS), and function score (FS) were also not significantly different. There was no significant difference in 30-day, 90-day, or 1-year mortality for bilateral TKR versus unilateral TKR (Table 1).

The 30-day, 90-day, and 1-year mortality in Hong Kong public hospitals was 0.1%, 0.2%, and 0.7%, respectively. These compared favourably with data of large national joint registries of other countries: 0.2% to 0.4%, 0.4% to 0.7%, and 1% to 2%, respectively.1 2 3 4 5 6 7 8 There is no definitive explanation for such findings but several possibilities exist. First, TKR is still mostly considered a ‘risky’ and major operation in Hong Kong such that the popularity of such surgery remains low compared with other countries. In 2013, the incidence of primary TKR was around 4 per 10 000 population in Hong Kong (estimated from data of the present study) compared with 12 in the United Kingdom, 14 in Sweden, and 19 in Australia.13 14 15 Lower operation incidence implies stricter selection criteria for operation. Second, the mortality of the general population of Hong Kong is known to be among the lowest in the world16; our findings may partly reflect the low mortality of the general population. Third, easy access to medical treatment in Hong Kong might facilitate timely intervention of early complications, hence reducing postoperative mortality. Whatever the explanation, the lower mortality indicates that primary TKR in Hong Kong are safe and conform to international standards.

One-year mortality following primary TKR in Hong Kong was lower than the mortality of the general population of the same age16 (Table 1). Similar findings have been shown by other studies.17 It has been suggested that strict selection criteria for operation meant that those selected were of better health than the general population. It was also hypothesised that pain relief and restored function would have a positive effect on a patient’s overall health, hence a lower mortality in the long term. Nevertheless, 1-year may be too short a period for the latter effect to be obvious.

In the present study, older age at operation was identified as a significant risk factor. This is consistent with findings in other studies.2 3 4 6 7 8 9 11 18 Some studies have reported higher 30-day,2 3 4 higher 90-day,6 7 and even higher total mortality in the long term.9 11 With an ageing population and higher life expectancy, there will be more patients with older age in future who undergo TKR. To date, there is no consensus on an age limit for the procedure. It is agreed that patients in their 80s or even 90s could still benefit from the surgery18 provided the associated higher mortality is well explained and accepted.

The presence of co-morbidities was not a significant predictor of mortality in the present study. Rather, the poor control or the severity of co-morbidities in terms of ASA class 3 was found by univariate analysis to be a significant factor. There is evidence that patients with only specific co-morbidities such as cardiovascular disease will have higher mortality.1 4 6 7 8 19 In addition, higher 30-day mortality,3 90-day mortality,6 and total mortality8 9 have been associated with higher ASA class. The lack of significance of ASA class 3 in multivariate analysis in our study suggests an underlying confounding factor. Analysis by t test showed that the age of patients with ASA class 3 was significantly older (72 vs 67 years, P<0.001). Thus in the present study, ASA class was confounded by age at surgery.

Preoperative ROM was also found to be a significant factor by univariate and multivariate analyses in the present study. This might be a novel finding. The exact explanation for such an association requires exploration by further study. One possibility is that preoperative ROM predicts postoperative ROM20 that in turn affects postoperative ambulation and function. As mentioned above, it was hypothesised that restored ambulation and function can have a positive effect on a patient’s overall health, hence a lower mortality. There are studies which reported an association between mortality and postoperative knee function. The latter was in terms of preoperative ambulatory status,8 postoperative ambulatory status, and postoperative WOMAC pain score.11 No knee score in the present study was found to be a significant predictor of mortality, however. One explanation could be that FS, WOMAC, and a large portion of KSS are patient-reported outcomes whereas ROM is an objective measurement; the more objective the measurement, the better it might be in predicting a secondary outcome such as mortality.

Although bilateral TKR has been found by several studies to have a higher mortality rate,12 21 22 it was not a significant predictor in the present study. Our institute performed bilateral TKR in selected patients with mild and well-controlled co-morbidities and younger age. Also, the fast-track rehabilitation protocol was used with an average length of hospital stay of 9.6 days (authors’ unpublished data). The results of analysis might reflect the equivalent safety of bilateral TKR with careful patient selection and fast-track rehabilitation. Many studies have demonstrated equal mortality for bilateral TKR with careful patient selection and a fast-track protocol.23 24

The present study has some limitations. First, due to inconsistent documentation across all public hospitals, data for case-control analysis for predictors of mortality were obtained from patients at our institute only. The smaller sample size limited the power of analysis of the present study. Second, since 70% of the mortality of our institute occurred more than 1 year after surgery and the mean operation-to-death interval was 21 months, analysis for predictors of mortality was performed on total mortality rather than 30-day, 90-day, or 1-year mortality. The very low early mortality in our institute and in Hong Kong means that a more powerful analysis of mortalities within 1 year may require a much larger sample size. This calls for a citywide joint replacement registry in which there is unified and detailed documentation of preoperative patient parameters, operative details, and postoperative outcome measurements. Such registries have already been established nationwide for 11 years in the United Kingdom13 and for 40 years in Sweden.14 Third, data for other known significant predictors of all-cause mortality, such as smoking and alcoholism, were not analysed. These factors might have confounded the present study. Since these factors are not known to be associated with age or preoperative ROM, the influence of these potential confounders on the conclusion of the present study should be insignificant. Lastly, the period chosen for selection of the control group did not fully match the death cases. This may have introduced confounding factors or made the groups incomparable. Since there was no change in the indications for surgery, surgical practice or rehabilitation protocol during the study period, and the sampled control should be representative of the target population.

Mortality after primary TKR was low in public hospitals in Hong Kong. Patients of older age or poorer general health in terms of poor ROM or ASA class 3 should be in optimal health before surgery and counselled about the higher mortality rate. The role of a pre-admission clinic and fast-track rehabilitation in contributing to the lower mortality in our institute should be further explored. A citywide joint replacement registry may help monitor and analyse post-TKR mortality specific to our locality.

All authors have disclosed no conflicts of interest.

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